1) Field of the Invention
The present invention relates to an inspection method for a multilayer gas sensing device installed in an exhaust system of an internal combustion engine for combustion control.
2) Description of the Related Art
The following multilayer type device has been known as a multilayer gas sensing device to be provided in an exhaust system of an engine of a vehicle to measure the concentrations of various gases contained in an exhaust gas, with the measurement results being used for engine combustion control or the like.
As shown in FIG. 11, this multilayer gas sensing device, generally designated at reference numeral 1, is made up of a solid electrolyte plate 11 carrying a measured gas side electrode 121 exposed to a measured gas (gas to be measured) and a reference electrode 131 confronting a reference gas chamber 150 and exposed to a reference gas, a spacer 15 for defining the reference gas chamber 150 and a heater substrate 191 (which will be described later).
In this multilayer gas sensing device 1, the measured gas side electrode 121 is covered with a porous diffusion resistance layer 141, and a dense protective layer 144 capable of substantially cutting off gases is provided to surround the overall circumference of the multilayer gas sensing device 1.
Meanwhile, a multilayer gas sensing device is produced by properly stacking green sheets forming a solid electrolyte plate, a spacer and others and calcining the resultant stack. Therefore, if the adherence between the green sheets is insufficient, off-specification products can develop. Moreover, there is a possibility of the occurrence of cracking in the green sheets, the electrodes and others. For searching these defectives, a conventional technique has employed a staining inspection.
However, in the case of the staining inspection, if the staining solution do not soak sufficiently into adherence-insufficient portions or cracking portions, difficulty is experienced in detecting troubles.
In addition, since the multilayer gas sensing device constructed as mentioned above is covered with a dense protective layer throughout its overall circumference, the staining solution soakage insufficiency occurs in the adherence-insufficient portions or the cracking portions, which makes it very difficult to find the defective products.
Still additionally, for example, in the case of the constructions shown in FIGS. 3 and 4 other than the construction shown in FIG. 11 which have a dense protective layer formed on the surface of the multilayer gas sensing device, likewise, the staining solution soakage insufficiency occurs, which makes it very difficult to find the defective products.